The present invention relates to a nonvolatile memory using ferroelectric capacitors. More particularly the present invention relates to a highly reliable nonvolatile memory which use ferroelectric capacitors that are free from fatigue due to read and write of data.
Memories using ferroelectric materials or ferroelectric random access memories (FERAMs) are nonvolatile memories which store data by polarizing the ferroelectric material in different directions. The use of FERAMs provide unique advantages, one being that data stored in the ferroelectric material is not lost when power is no longer available to drive associated memories.
In a conventional ferroelectric memory, polarization is switched when data is read and written. A ferroelectric film forming the ferroelectric memory is fatigued in every writing or reading operation. Consequently, there is a problem that the allowable number of times of write and read operations is extremely restricted in comparison with, for example, a dynamic random access memory. Moreover, since it takes a relatively long time to switch the polarization, there is a problem that the operation time is made longer by the polarization switching.
In order to solve the problems associated with deterioration of the ferroelectric film and the reduction in the reading rate resulting from the switching of polarization, the following method has been proposed in Japanese Patent Laid-Open Publication No. 283176/1991. As shown by an array configuration of FIG. 15, the potential of the plate line is set to, for example, power supply potential (Vcc), and the memory is used as a DRAM in a normal operation. Further, data is stored as nonvolatile information by a FERAM write operation before the power supply is turned off. With the plate line thus set to Vcc, data is prevented from being switched in either case where the potential of a storage unit is ground potential (0) or Vcc. Therefore, the problem of deteriorating the ferroelectric capacitor is avoided and the reading rate is not reduced. If the nonvolatile data is read through the FERAM operation when the power supply is turned on, the FERAM may be made to function effectively as a nonvolatile memory.
However, there still exists a problem, in the above described method when the memory is operated in both DRAM and the FERAM modes. That is, the operation of converting volatile data to nonvolatile data tends to be complicated. More specifically, data is read through the DRAM operation, first, and then the data requires storing as nonvolatile data correspondingly through the FERAM operation with respect to all the memory cells. Particularly when the power supply is turned off by accident, it is extremely difficult to terminate the above conversion operation quickly. Since the polarization of ferroelectric materials is aligned unidirectionally during the use of the memory cells as a DRAM in the above method, all the stored data may be lost when the power supply is turned off by accident.